1. Field of the Invention
The present invention relates generally to the field of telecommunications and, more specifically, to programmable telecommunications switches having improved redundancy features.
2. Discussion of the Prior Art
Programmable switches for switching time division multiplexed (TDM) telecommunications signals are used in a wide variety of applications such as voice messaging, telemarketing services and the like. A programmable switch is usually controlled by a host device, which is typically a computer that runs a telecommunications application program. A customer may either purchase a commercially available application program that is compatible with the host and switch hardware, or may elect to write a custom program.
A modern telecommunications switch is typically organized with voice and data traffic all being routed along a central TDM bus. Interfaces to the public switched telephone network, (PSTN) are grouped onto circuit boards or "line cards" along with control circuitry for handling the traffic coming through that card. Different line cards may include different interfaces for supporting various signalling protocols, including T1, E1 and J1. In any system, there may be a combination of line cards each of which may carry T1, E1, J1, DS3, SONET or analog signals.
All of the line cards receive instructions from a central processing unit (CPU), often a microprocessor, along a high level data link control (HDLC) bus. Each line card has a designated slot in a passive backplane or motherboard into which it mounts, and a connector which establishes the appropriate signal paths between the card and the TDM bus when the card is mounted in the slot. Incoming communications signals are transmitted onto the TDM bus via the line cards, and are switched to the appropriate outgoing port which may or may not be located on another line card. All of the traffic on the TDM bus is controlled by the CPU, which responds to messages from the host. The CPU controls the routine switching of the voice and data traffic through a switching matrix, typically a time slot interchange.
Usually a switch has a number of line cards, all transmitting data simultaneously. If one of the line cards should fail, the data stream which was previously channeled through that card must be rerouted through a backup card by replacing it with a new card. Otherwise, switching to the spans supported by the failed card are completely lost.